As the requirements for the capacity and flexibility of the communication system are gradually improved, the coherent communication technique becomes more and more important. In comparison with non-coherent technique (e.g., on-off key, OOK) or self-coherent technique (e.g., differential quadrature phase shift key, DQPK), the coherent technique has the following advantages: the signal to noise ratio (SNR) gain is 3 dB; transmission capacity can be improved by using a more effective modulation technique (e.g., quadrature modulation, QAM), and an Electronic Equalization technique can be conveniently used to deal with the channel change and reduce the cost, etc. Presently, the recovery of carrier phase in the coherent receiver is generally realized via the digital technique, such as a method based on the m-th power, proposed by D. Ly-Gagon, etc. in “Coherent detection of optical quadrature phase-shift keying signal with carrier phase estimation” (Journal of Lightwave Technology, Vol. 24, No. 1, January 2006, pp. 12-21), and a method based on data pre-decision, proposed by Z. Tao etc. in “Multiplier-free Phase Recovery for Optical Coherent Receiver” (OWT2, OFC2008). The common ground between digital phase recovery techniques is that: the carrier phase is assumed invariant at multiple data symbols, so as to eliminate the influence of channel noise on the phase estimation using an average of the carrier phases on the symbols. Such assumption apparatus that, in order to get an optimum phase recovery performance, the average length shall be an optimum compromise between the intensity of channel noise and the variation speed of carrier phase. Multiple tests have shown that an unmatched average length has a great influence on the performance of the coherent receiver.
However, in actual transmission system, the intensity of channel noise and the variation speed of carrier phase are dependent on many factors (e.g., characteristics of the laser and channel), but those factors are variable and difficult to be measured via real-time detection.
It can be seen that the average length in digital phase recovery shall be optimized, so that the coherent receiver can work in an optimum state. This technique must have the following characteristics:    1. Non-training: prior knowledge such as training sequence or channel/laser characteristics is not required;    2. Adaptability: optimization can be automatically realized with changes of the channel noise intensity and the variation speed of carrier phase;    3. Simple calculation: the calculation is of low complexity so as to be realized in high speed receiver.
In order to solve the above problems, the present invention provides a method and apparatus for automatically optimizing an average length used in a phase recovery device.